The study of mechanics of pulmonary capillaries and their role in pulmonary circulation has been limited by the techniques available and by the difficulty of making direct measurements. Accodring to the Fahraeus effect, the hematocrit and hence the gravimetric density of capillary blood is lower than the circulating blood. Regarding the density difference as a density indicator, we hypothesize that a change in the blood volume of pulmonary capillaries becomes equivalent to an injection of the density indicator into the blood flow. Using an accurate density meter, we measure density changes in the blood outflowing from the lung to assess the changes in pulmonary capillaries. Our long term objective is to use this method to study how mechanical, physiological, and pathological changes of pulmonary capillaries affect the circulation of the lung. The first specific aim of our research is directed to further verify this hypothesis by demonstrating the equivalence of the ether transit time with a delay time derived from the density method and by testing the implications of the hypothesis with hemodilution. Second specific aim is to analyze the density fluctuation with the theory of indicator dilution to quantify the compliances of pulmonary capillaries due to loading in airway and vascular pressure. Our third specific aim is to use the density methodology to monitor the change in density due to bead infusion and PEEP. An analysis on the density with bead infusion allows us to quantify the fraction of blocked capillaries, the pressure gradient in normal pulmonary capillaries, the collateral flow, and the effect of embolism on pulmonary circulation. We evaluate the density transient induced by PEEP to identify its effect on pulmonary capillaries and the mechanics of capillary recruitment.